Superheterodyne and direct conversion receivers and waveform generators (also known as exciters) are employed in a wide range of applications. These include (but are not limited to) radar and communications transceivers. These receivers provide the ability to select a narrower RF band within a wider operational band for signal reception and transmission using detection and modulation circuits implemented at narrower intermediate frequencies bands, which are typically lower than the transmit band frequencies, and include baseband. The narrower or lower center frequency intermediate frequency bands are better suited to detection and modulation where analog-to-digital converter (ADC), and digital-to-analog converter (DAC) performance is improved and more commonly available. The frequency conversion is accomplished through at least one or more stage of mixing for up-conversion and/or down-conversion. An ability needed for these receivers and exciters is for frequency selectivity and/or hopping to provide frequency agility within the operational band. Agility relates to both the number of frequencies covering tuning over an operational band and also relates to the speed of tuning. The local oscillator synthesizers employed produce the reference signals at an offset from the radio frequency of interest, where the offsets are related to the intermediate frequencies. The synthesizer typically adjusts its frequency to address the agility of the receiver, but also generates phase noise and spurious signals which degrade the performance of the receiver and exciter.
To address suppression of clutter in radar, and generally the suppression of interference and jamming in transceivers, the receiver dynamic range and/or the waveform generator's spectral purity have certain requirements. With advancing threats and denser RF environments, the ability of prior art frequency agile synthesizers are limited by phase noise and spurious produced and are also more complex than desired. In particular higher performance low phase noise agile local oscillator (LO) synthesis results in configurations with significant increases in overall complexity, size weight and power consumption.
The phase noise and spectral purity of LO synthesizers are derived by employing low phase noise fixed frequency oscillators employing high quality factor resonators for which all synthesized frequencies are synchronized to in order to meet particular frequency accuracy, drift and phase coherence requirements. Agile high performance synthesizers typically degrade phase noise by 20 dB to 40 dB over the best available fixed frequency oscillators for typical levels of agility desired.
The traditional high performance low phase noise LO synthesis employs direct RF synthesis techniques known within the prior art, and includes low phase noise frequency division, mixing, and switched filtering to develop an ultra low noise frequency agile LO generated by combining a smaller number of frequencies in stages. This approach, however, has phase noise limitations due to the addition of multiple components serially within the chain, which add residual phase noise to the output signal. In practice receivers and waveform generators having wide operational bands and many center frequencies required are limited in phase noise performance by the synthesizer by a significant margin over the best oscillators.
Additionally multiple bandpass filters are utilized to reject mixer and harmonic spurious, where settling time in switching filters, and related complexity of multiple filters, switches and loss recovery amplifiers increases size, weight and power consumptions while further limiting phase noise.
Frequency synthesis techniques, such as those that use phase locked loop (PLL) employ frequency fractional division devices within the PLL feedback loop for frequency agile synthesis and are subject to both settling time limitations, and spurious and noise generated in the fractional dividers, which are subsequently amplified at the output.
Direct digital synthesis (DDS) employing digital-to-analog (DAC) devices, have lower than desired linearity performances resulting in generation of unwanted spurious signals due to amplitude and time skew mismatch distortion effects and due to phase accumulator phase quantization resulting in a large number of spurious signals.
Recently, there has been focus brought to architectures and technologies that combine a plurality of LO synthesizer units to yield an improvement of phase noise relative to a single unit. While this approach can reduce uncorrelated noise terms it does not address harmonic or other correlated terms. A simplistic approach results in cost, size and weight of a plurality of traditional high performance LO synthesizers can be prohibitive. Other approaches employing miniaturized tunable oscillators arrays on integrated circuits with frequency injection locking address the size weight and power and complexity of parallel architectures, but performance of the individual synthesizers are limited in phase noise and agility.
Accordingly, an improved frequency LO synthesizer is desirable in the art that provides improved spectral purity, improved frequency agility, in addition to size weight and power advantages.